OLIVE : MONOGRAPH OF THE ACRASIEAE. 
485 
pseudoplasmodinm stage. But with the formation of cellulose walls, 
the colony slowly assumes the characteristic color, the purplish 
forms, when in a young condition, showing a reddish tinge, later 
becoming dark violet or purplish. Tests demonstrate the fact that 
a similar change in the color of the dark spored fructifications can 
be brought about by varying the acidity or alkalinity, so that the 
color changes to reddish in acids and resumes the bluish tinge on 
ajDplying a dilute alkaline reagent. 
Further than the fact that the color seems to be associated with 
the cellulose walls, nothing definite was determined concerning the 
phenomenon in the three species of the Dictyosteliaceae examined, 
namely, Polysphondylixim violaceiim, Dictyostelium puTpiireum^ and 
D. aureum. Van Tieghem makes the statement that the violet 
color in the case of Acrasis is associated with the cuticularized 
external portion of the spore ; and he further describes a peculiar 
condition in Coenonia, in which the cells of the stalk are colorless, 
while the cellulose walls of the sj^ores in the sorus are yellowish. 
Irkitability of the Pseudoplasmohium. 
Certain phenomena connected with the irritability of these fructi¬ 
fying masses are of sufficient interest to merit a special discussion. 
The plastic masses of naked protoplasm which form the pseudoplas- 
modia of the higher forms, furnish excellent illustrations for the 
study of the irritability of such fruiting bodies with respect to cer¬ 
tain stimuli. That which renders these phenomena more interest¬ 
ing is the fact that here we have to deal structurally with an aggre¬ 
gation of individuals, each to a certain extent distinct, while physio¬ 
logically the colony acts as a unit with respect to stimuli. The 
response of the pseudoplasmodinm is similar in most respects 
to that of the sporophores of certain Myxomycetes and of fungi, 
such as Pilobolus. For example, young pseudoplasmodia of Dicty- 
ostelium and Polysphondylium are strongly heliotropic, so that if a 
culture tube be turned until the nutrient surface is parallel to the 
source of light, the colony will turn so as to be almost parallel to 
the substratum. In general, the direction taken is a resultant 
between that induced by the heliotropic force and that which 
causes the pseudoplasmodinm to turn at right angles to the agar 
